Bleaching of cellulosic materials



Patented Nov. 30, 193" BLEACHING OF CELLULOSIC MATERIALS Maurice C. Taylor and James F. White, Niagara Falls, N. Y., assignors to The Mathieson Alkali Works, Inc.., New York, N. Y., a corporation of Virginia No Drawing. Application February 18, 1936, Serial No. 64,458

Claims.

This invention relates to improvements in the bleaching of cellulosic materials, more particularly of materials composed of cellulose or of cellulose derivatives. The invention involves a novel 5 application of the salts of chlorous acid for this purpose. This application is in part a continuation of application Serial No. 445,196 filed April Chlorous acid, H0102, forms salts with alkalimetals, such as sodium chlorite, NaClOz, and the alkaline-earth-metals, such as calcium chlorite, Ca(ClOz)2.'

These salts, chlorites, as applied to the bleaching of cellulosic material, difler in important respects from other bleaching agents of the class comprising chlorine and the hypochlorites. Chlorine and the hypochlorites are among the agents more commonly used for the bleaching of cellulosic materials and they are very efiective agents for this purpose. Their cost is low and their action rapid. However, bleaching agents of the class comprising chlorine and the hypochlorites, tend to exert a destructive action upon the cellulosic material itself when an attempt is made I to secure maximum bleaching results. This tendency has involved, in one aspect, critical control of bleaching operations in which bleaching agents of this class are used, and in another aspect, reduction of the strength of the cellulosic material as well as actual loss of the cellulosic material itself.

We have discovered that the chlorites are eifective bleaching agents but that, with the chlorites as distinguished from bleaching agents of the class comprising chlorine and the hypochlorites, any tendency to exert such destructive action upon the cellulosic material itself either does not exist or is for practical purposes, negligible. Consequently the application of the chlorites to the bleaching of cellulosic material involves neither the necessity for critical control nor loss of or damage to the cellulosic material itself. In this application of the chlorites, factors such as temperature, concentration and acidity or alkalinity either are not critical or are much less critical.

The bleaching of cellulosic material with the chlorites may be carried out under acid, neutral or alkaline conditions. The temperatureis not critical; in general increasing the temperature accelerates the bleaching action. The bleaching of cellulosic material with the chlorites may be carried out at a temperature approximating 100 C. without destructive action on the cellulosic material and without appreciable loss of available oxygen. Higher temperatures may be used by carrying out the operation-under superatmospheric pressure. Since the chlorites are so inert withrespect to cellulosic material the initial and final concentrations of the chlorite may be high or low, without destructive action on the cellulosic 5 material or without prejudice to the bleaching operations. The period of contact between the chlorite and the cellulosic material also is not critical. It is important that suflicient time be allowed for the bleaching operation to proceed to 10 the desired point but the period of any continued contact beyond this point is unimportant. The bleaching operation may thus be carried out under the conditions best meeting the economic requirements in each particular case. The action of the 15 chlorites in bleaching cellulosic materials is however somewhat slower than that of the more generally used bleaching agents, such as chlorine and the hypochlorites, which tend to exert a destructive action on the cellulosic material as the 2 point of maximum bleaching is approached. Also the cost of the chlorites is somewhat higher.

According to our present invention, cellulosic material is bleached by subjecting it to treatment with any of the bleaching agents of the class 5 which tends to exert a destructive action on the cellulosic material, the severity and duration of said treatment being limited to avoid destructive action on the cellulosic material, and by subjecting it thereafter to treatment with an aqueous 30 solution containing-a chlorite, a chlorite of the alkali-metals or a chlorite of the alkaline-earthmetals for example. By this improved method of bleaching, cellulosic materials may be treated to secure maximum bleaching results without un- 35 due expense or undue prolongation of the bleaching operation and without the necessity of critical control as the point of maximum bleaching is approached. The initial bleaching step, as with chlorine or a hypochlorite, may be carried out 40 in any conventional manner. Following the initial bleaching step the bleaching is completed with an aqueous chlorite solution, advantageously with an acid aqueous chlorite solution. Aqueous chlorite solutions having a pH approximating 5 3.5-5.0 are particularly advantageous. The chlorite bleaching step maybe carried out over a wide temperature range including 20 C., 50 (3., 0., 100 C.,- etc. However temperatures between 50 C. and C. are particularly advan- 50 tageous. Concentration of the treating solution may vary widely as the salts are very soluble.

The improved bleaching method of our invention is applicable generally to wood pulps intended for the manufacture of paper, including wood pulps produced by the kraft process, by the soda process and by the sulflte process. It is also applicable to other cellulosic materials such as cotton, rag pulp, rayon and hemp. The application of the bleaching method of our invention to wood pulps produced by the kraft process and by the suliite process as well as to cotton, rag pulp and hemp will be illustrated by the following examples: a

' Example I A kraft pulp having a bleachability of 17.6% available chlorine was subjected, in a beater, to

the action of 10%, on the ,bone dry-weight of the pulp, of chlorine as such while maintaining a temperature approximating 25 C. Chlorine was added progressively 'and, during the chlorine addition, milk of lime was added as required to maintain the pulp suspension very slightly acid. The incompletely bleached pulp was then thoroughly washed and after washing had a residual bleachability of 5.95% available chlorine. The washed pulp was diluted with water to a pulp density of 5% and the pH value was adjusted, with hydrochloric acid to approximate 4.5. The bleaching was then completed, while maintaining active agitation and a temperature of 70- C. by adding of the residual bleachability, that is 6.55% on the bone dry weight of the pulp, of available chlorine as sodium chlorite. A satisfactory color was attained in three hours without impairment of the strength of the pulp. Following the initial bleaching with chlorine the incompletely bleached pulp may be subjected to treatment with an alkali, such as caustic soda or lime, in aqueous solution and then washed or, as in the foregoing example, the incompletely bleached pulp may be washed without being subjected to such treatment with an alkali.

The following table of properties compares, in each instance, unbleached kraft pulp with the same pulp bleached by the procedure just described with chlorine followed by sodium chlorite in accordance with this invention, and with the same pulp subjected to a standard chlorine perature of 255 0., 2.21% of available chlorine on the bone dry weight of the pulp, was'added as water saturated with chlorine. At the end of six minutes, 2.65% of lime, on the bone dryv weight of the pulp, was added as a concentrated aqueous. slurry, agitation being continued. At the end of an additional five minutes, the chlo -frine was 98% consumed. The pulp was'then thoroughly washed. The washed pulp was diluted with water to a pulpdensity of 5% and the pH value was adjusted, with hydrochloric acid to approximate 3.8. The bleaching was then completed, while maintaining active agitation and a temperature of 50 C., by adding 1.5% of available chlorine on the bone dry weight of the pulp as sodiumchlorite. A satisfactory color was attained in 2.5/ hours. The samples to which the following tableof properties refers were subjected to this chlorite bleaching operation for an additional period of 2.5 hours making a total of 5 hours.

Example III A harder cooked sulflte pulp with a bleachability of 4.45% of available chlorine was diluted with water to a pulp density of 2.9%. While maintaining active agitation and a temperature of 225 C. 3.0% of available chlorine, on the bone dry weight of the pulp, was added as water saturated with chlorine. At the end of six min.- utes 3.6% of lime was added as a concentrated aqueous slurry, agitation being continued. At the end of an additional five minutes the pulp was thoroughly washed. The washed pulp was diluted with water to a pulp density of 5% and the pH value was adjusted, with hydrochloric acid, to approximately 4.0. The bleaching was then completed while maintaining active agitation and a temperature of 50 C. by adding 1.2% of available chlorine, on the bone dry weight of the pulp, as sodium chlorite. A satisfactory color was attained in 3.5 hours.

The following table of properties compares, in each instance, the unbleached pulp with the same pulp bleached with calcium hypochlorite bleach' and the same pulp bleached with chlorine followed by sodium chlorite in accordance with this Chlorine i n Um iollgwed st n d rd nven Pulp am ml... the

chlorite Chlorine Um Calcium followed Easy bleaching sulilte pulp bleached hypoby Belisweight480sheets24x36.--.-. 21.5 -ao.4 31.0 chlorite ig Bursting stre h p cl-cent (Mullen) h 117 87 c o e Tensile strongt ,l (Schoppel'): 14 3 15 2 13 8 M a direct 6: 4 5: 5 5: 0 Folding endurance 660 473 10 (Double folds) 2. 6 2. 4 1. 7 Bursting strength 89 90 107 4.2 4.5 a0 (Mullen) Percent alpha cellulose 87.27 as. 11 s1. c0 s3 54 4e Viscosity 01 19.6 03 54 (is 46 (Centigoises) 100 84 46 Percent ot alkali soluble 22.58 22.40 21.51

. oval 0035 0035 Copper number 1. 94 2. 2 1, 7

The stock bleached with chlorinefollowed by so- CB] 1 91 dium chlorite was much. whiter than the stock Um cu l oiowed which had been subjected to the standard chlo bleached 3%,, rine bleaching treatment, although, as shown by lorite the foregoing table, the former was much superior to the latter in strength and was in fact. Folding endurance 930 550 m substantially equal to the unbleached stock in ggg g gg ggl 108 m s P t l hacellulosc e409 8304 8557 Ew'wle II vtfii yf n-....-::::::::::::: 81' 24:5 24' An easy bleaching sulflte pulp having a blea Prcgi i t 3: alkali soluble 23.10 23.04 22. 15 ability or 3.6% available chlorine, was diluted m numb" 1J1 with water to a pulp density of 2.9% (by weight).

While maintaining active agitation and a tem- Hypochlorite can be substituted for chlorine in the foregoing examples using an amount of hypochlorite equivalent to the'chlorine in oxidizing value. However, the chlorine appears to give a pulp of superior quality.

Example IV Cotton llnters were treated, at room temperature, with a solution of calcium hypochlorite containing about .03% to .05% of available chlorine. After sufficient time had elapsed to allow 50% to 95% of the available chlorine to be consumed, the cotton was washed. A. suspension of approximately 2% to of the cotton in water 20 grams 01' rag pulp containing approximatel 50% moisture were suspended in 600 ccs. of a solution containing 0.20 gram of available chlorine as calcium hypochlorite. This was allowed to react at room temperature, with stirring, for 24 hours after which time the available chlorine was exhausted. The pulp then was washed with water. The washed pulp then was subjected to a second treatment in 600 ccs. of a solution containing 0.40 gram of available chlorine as sodium chlorite. For the chlorite treatment the temperature was raised to 80 C. and the pH value of the solution adjusted to 3.5. At the end of 16 hours of this treatment the available chlorine had been exhausted and the pulp was drainedand washed with water.

. Example VI 20 grams of rag pulp containing approximately 50% moisture were suspended in 600 cos. of chlorine water containing 0.20 gram of chlorine. This was allowed to react for 24 hours at room temperature after which the pulp was drained and washed with water. The pulp was then subjected to treatment at a temperature of 80 C. for 17 hours in 600 ccs. of a solution containing 0.40 gram of available chlorine as sodium chlorite. The solution had a pH value of 3.5. The

pulp was then drained and washed with .water..

Example VII 96 grams (bone dry) of whole stock of hemp, after cooking, were treated at a 3% consistence with 1.06 grams of chlorine, as chlorine water, for 15 minutes at a temperature slightly below 20 C. This was neutralized with 1% grams of sodium hydroxide and after 5 minutes the pulp was washed. The washed pulp was diluted to a 5% consistence and then treated with a total of 5.7 grams of available chlorine as sodium chlorite. The sodium chlorite treatment was carried out at a temperature between 60 C. and 75 C. and at a pH 3.5-4.0 for 3 hours. The pulp was then treated with acidified bisulflte and washed. This treatment gave a white pulp of exceptionally high alpha-cellulose content.

While the chlorites oi the metals of the class comprising the alkali-metals and the alkalineearth-metals are particularly useful in carrying out our invention, the salts of chlorous acid generally as well as chlorous acid also are useful since the chlorite radical is the component important with respect to our invention. It will be appre- 'ciated that the chlorite should be chosen for any particular purpose to avoid undesired reactions such as the precipitation, if that is undesirable, of calcium sulfate through the use of calcium chlorite, rather than sodium chlorite for example, in a solution also containing a sulfate.

We claim:

1. The method of bleaching cellulosic material whichcomprises subjecting the cellulosic material to an initial treatment with a bleaching agent from the class consisting of chlorine and the hypochlorites, which bleaching agent will act upon the coloring matter carried by the cellulosic material and which, if maximum bleaching were obtained with it, would act upon the cellulosic material with destructive action thereon, permitting such treatment to proceed until substantial action on the coloring matter hastaken place, discontinuing said initial treatment before substantial destructive action on the cellulosic material has taken place, and thereafter subjecting the thus treated cellulosic material to a bleaching treatment'with an aqueous acid solution containing a chlorite of a metal of the class consisting of the alkali metals and the alkalineearth-metals, whereby the cellulosic material will be bleached without substantial destructive action.

2. The method of bleaching cellulosic material which comprises subjecting the cellulosic 'material to an initial treatment with a bleaching agent from the class consisting of chlorine and the hypochlorites, which bleaching agent will act upon the coloring matter carried by the cellulosic material and which, if maximum bleaching were obtained with it, would act upon the cellulosic material with destructive action thereon, permitting such treatment to proceed until substantial action on the coloring matter has taken place, discontinuing said initial treatment before substantial destructive action on the cellulosic material has taken place, and thereafter subjecting the thus treated cellulosic material to a bleaching treatment with an aqueous solution containing a chlorite of a metal of the class consisting of the alkali metals and the alkalineearth-metals, whereby the cellulosic material will be bleached without substantial destructive action.

3. The method of bleaching cellulosicmaterial which comprises subjecting the cellulosic material to an initial treatment with a bleaching agent from the class consisting of chlorine and the hypochlorites, which bleaching agent will act upon coloring matter carried by the cellulosic material and which, if maximum bleaching were obtained with it, would act upon the cellulosic material with destructive action thereon, permitting such treatment to proceed until substantial action on the coloring matter has taken place, discontinuing said initial treatment before substantial destructive action on the cellulosic material has taken place, and thereafter subjecting the thus treated cellulosic material to a bleaching treatment with an aqueous solution containing a chlorite of a metal of the class consisting of the alkali metals and the alkaline-earth metals having a pH value approximating 3.5-5.0 at a temperature upwards of about 50 0., whereby the cellulosic material will be bleached without substantial destructive action.

4. The method of bleaching cellulosic material which comprises sub, .cting the cellulosic material to an initial treatment with chlorine. rial to an initial treatment with a hypochlorite.

permitting such treatment to proceed until substantial action on coloring matter carried by the cellulosic material has taken place, discontinuing said initial treatment before substantial destructive action on the cellulosic material has taken place, and thereafter subjecting the thus treated cellulosic material to a bleaching treatment with an aqueous solution containing a chlorite of a metal of the class consisting of the alkali metals and the alkaline-earth-metals, whereby the cellulosic material will be bleached without substantial destructive action.

5. The method of bleaching cellulosic material which comprises subjecting the cellulosic matelulosic material will be bleached without substantial destructive action. I

MAURICE C. TAYLOR.

JAMES F. 

